A manipulator which comprises more than one arm and where at least two arms each form a chain of joints between the actuators of the manipulator and the platform which is to be manipulated is called a parallel kinematic manipulator. For a fully built-up parallel kinematic manipulator for movement of a platform with three degrees of freedom (e.g. in directions x, y and z in a Cartesian system of coordinates), three parallel-working arms are required, and if all the six degrees of freedom of the platform are to be manipulated, six parallel-working arms are required. In many industrial applications where at present linear manipulators of a socalled gantry type are used, four degrees of freedom are normally required, which means that a corresponding parallel kinematic manipulator shall have four parallel arms.
To obtain a stiff arm system with a large loading capacity and a low weight, the lower arms of the parallel kinematic manipulator nearest the manipulated platform shall have a total of six links which only transmit compressive and tensile forces. For a manipulator for four degrees of freedom and four arms, this implies that the four lower arms must share the six links between them and this can only be done with certain combinations, such as, for example, 2/2/1/1 or 3/1/1/1. If one of the links is used for transmitting torque in addition to compressive and tensile forces, the following possible combinations for a parallel kinematic manipulator with four degrees of freedom are also obtained: 3/2/1, 2/2/2. These combinations may also be used when only three degrees of freedom are to be manipulated in the manipulated platform.
When a rectangular working range is required in manipulator applications, so-called gantry manipulators are used today. These manipulate a platform with normally four degrees of freedom: x, y, z and rotation around the z-axis. To this end these manipulators are composed of one axis of rotation and three series-connected linear paths, on which movable units are moved in the x-, y- and z-directions. The first movable unit, which is moved along a first linear path of an actuator, supports a second linear path mounted perpendicular to the first linear path. On the second linear path, there is then a second movable unit which in turn supports a third linear path, which is mounted perpendicular to both the first and second linear paths. On the third linear path there is a third movable unit, which supports an axis of rotation when the manipulator has four degrees of freedom.
The series connection of the linear paths with their associated movable units and actuators impose a number of restrictions on current gantry manipulators.                The manipulator becomes very heavy, which limits its speed of action and results in a need of expensive and energy-consuming actuators (motors).        The manipulator becomes weak and when objects or tools are moved, a undesired oscillation of the manipulator is obtained in case of movement along the path where the movement is to be made, and especially when the movement is to be stopped, so-called overshoots are obtained.        The manipulator becomes resilient when the platform is to generate forces between tools and objects, unless very expensive and complex solutions for the liner paths are used.        For the movable actuators with their associated measuring sensors, movable cabling is required, which causes poor reliability in gantry manipulators.        It is difficult to obtain high accuracy of the manipulator without providing expensive solutions involving, for example, air bearings, which at the same time give the manipulator limited speed of action.        Two parallel linear paths are normally used for supporting the second linear path in the serial kinematic chain. This gives rise to an effect similar to that of a drawer in a chest of drawers getting wedged when being pushed it, and requires special, costly solutions to manage.        
All of these limitations when using gantry manipulators can be eliminated by a parallel kinematic manipulator which is driven by parallel-working linear paths, which do not need to support each other but where all the paths may be mounted on a fixed frame structure. An example of such a parallel kinematic robot is Hexaglide, developed at the Technical Institute of Technology ETH in Zurich. The kinematics of this is clear from FIG. 1, which is described in the section DESCRIPTION OF THE PREFERRED EMBODIMENTS. Here, six movable units on three linear paths are used for guiding six degrees of freedom of the manipulated platform with the aid of six parallel links. This manipulator will have an arm system with a very low weight, will be rigid, may achieve large tool forces without yielding, has no movable cabling and may be given very high accuracy. However, this manipulator has too small a working range to replace the gantry manipulators that are used at present in various industrial applications. In addition, this parallel kinematic robot requires six actuators also when only four degrees of freedom are to be manipulated, which results in an unnecessarily high price of the manipulator. Finally, the control programs which are to attend to the movement of the manipulator become extensive.